Make code_magic into change_exchange

Remove my old unused maps.
Map to cx and cX and rename file to convey mnemonic.
Add more documentation.
Map key chords to navigating the jumplist

Since navigating the jumplist usually involves repeated operation, use
chording instead of paired keypresses. That's easier to hit. Keep the
old maps around for now.
sub: Add ConvertToPlaintextLossy and g to textconv

Adds new command ConvertToPlaintextLossy
Makes all replacements global (add g flag).